1. Field of the Invention
This invention relates to the construction of a substrate of a thermal recording head having a heat-generating resistance layer, a recording head having this substrate and further, a process for producing these, and a recording apparatus by use of these.
The present invention concerns a substrate for use in a liquid jet recording head which records images by causing a state change involving the formation of bubbles in a liquid by adding heat energy. This causes the liquid to be discharged through a discharge port to form flying droplets, which impinge on the surface to be recorded. A head for liquid jet recording is constructed using this substrate, and is suitable particularly for the multi-integration type liquid jet recording head.
The present invention is effective for use in thermal recording heads incorporated in printers, copying machines, facsimile machines, computer output instruments, etc.
2. Related Background Art
In the field of thermal recording methods, an effective substitute for thermal print (impact) method is the ink jet recording method, which as a non-impact method has recently attracted attention and has been practically applied.
All of the liquid jet recording methods described in, for example, Japanese Laid-open Patent Application No. 54-51837, and German Laid-Open Patent Application (DOLS) No. 2843064 have a specific feature different from other liquid jet recording methods in that the power source for causing droplet discharge is thermal energy, which is applied to the liquid.
More specifically, according to the recording method disclosed in the above-mentioned published specifications, the heated liquid undergoes a state charge accompanied by an abrupt increase of volume, and because of this state change, droplets are discharged and expelled through the discharge opening provided at the tip of the recording head to be printed on a recording medium material, thereby effecting recording of information.
The liquid recording method disclosed in DOLS No. 2843064, and U.S. Pat. Nos. 4,723,129 and 4,740,796 can be effectively applied to the so called drop-on demand recording method, but in addition the recording head can be easily designed with high density multi-discharge ports across its full line width, and therefore it has the advantage that high resolution images and high quality can be obtained at high speed.
The ink jet recording head based on such principle applies a voltage to the heat-generating resistor (heater) of the heat acting portion, and the resulting state change includes the formation of bubbles (the above-mentioned one discloses the preferable form of film boiling) on the heat acting surface, which acts on ink by the heat energy generated thereby, and the ink is expelled through the discharge opening by the the state change giving rise to such foaming. When the voltage is increased from zero level, foaming is initiated at a certain definite voltage. This certain voltage is important, and hereinafter is called the foaming voltage.
For discharging ink, a voltage greater than this foaming voltage (driving voltage) must be applied. Also, for improving printing quality, the driving voltage must be made higher than the foaming voltage, to some extent, while for improving pulse durability, the driving voltage must be minimized. The optimum value of those applied voltages has been standardized as corresponding to some multiple of the foaming voltage. Therefore, it is a very great factor in realizing improvement of printing quality, etc. how the foaming voltage which becomes the standard should be set.
More specifically, in order to obtain a uniform discharging characteristic/printing characteristic within the recording head, and also to improve discharging durability, it may be considered that the foaming voltage within the recording head should be always constant.
Whereas, for preparation of a thermal recording head, in which a plurality of heat-generating resistors, electrode pairs corresponding thereto and insullating protective layers are formed, film forming technique is practiced, but the problem of variations in the structure of the respective parts has occured in bulk production or from lot to lot. In practical application, in spite of these variations, the heat energy needed for ink discharge has been reliably provided by giving foaming voltage itself relatively over to great extent to the respective heat-generating resistors.
However, variations in electrothermal transducers including the respective resistors, electrodes and optional insulating layers become obstacles in improving printing precision.
One solution to this problem is the invention of Japanese Patent Application No. 60-297217 (Japanese Laid-open Patent Application No. 62-152863) filed by Canon K. K. as Applicant. This invention calls attention to the fact that all of the resistance layers, insulating layers, and electrodes become thinner at the both end regions as compared with the central region of the recording head when formed by sputtering, and has clarified that an electrothermal transducer with uniformized thickness can be obtained at the portions of concentric shapes. In this invention, since the range with relatively smaller variance is selected in the region to be formed into film, the electrothermal transducer cannot only accomplish linear higher densification, but also due to the difference of the recording gaps relative to the recording medium from each other, the whole recording must be uniformized by further control. Also, in this invention, it is difficult to obtain a full-line thermal head.
On the other hand, although U.S. Pat. No. 4,740,800 clearly describes that the width of the heat-generating resistance layer formed by etching is greatly varied, it only discloses as a solution that the center side with relatively less variance is used for recording without use of the heat-generating resistance layer on both end sides. Therefore, according to this invention, the recording head is enlarged, as will be the device. Of course, this invention may be practical for a recording head having less than a hundred and twenty-four, electrothermal transducers because no much enlargement is brought about, and is actually used. Anyway, for limited use of the range with relatively less variations, secondary control means for such variations is required, and the variations become greater in the case of one thousand or more electrothermal transducers formed into a full line.
Thus, in the prior art, because there has been no fundamental solution of the problems in production of thermal head, recording has been performed by selecting electrothermal transducers with relatively smaller variations.